The present invention relates to coulometric analysis which may be used to determine the quantity of a substance in a sample.
Coulometry is a well known analytical technique which under ideal conditions allows the direct quantitative determination of the mass of a substance in an analyte without it being necessary to calibrate the analytical equipment against a standard. Coulometry relies upon the measurement of the amount of electrical charge required to deplete a solution of a substance to determine the amount of that substance initially in the solution. If the electron stoichiometry of a reaction of interest is known, and there are no competing reactions, it is possible to determine the mass of a substance of interest simply by integrating current flow at an appropriate potential over time. Generally, a potential is applied between a working electrode and a reference electrode immersed in the analyte, and current is supplied to the analyte from a counter electrode to balance the current through the working electrode. The counter electrode current is a measure of the rate of depletion of the analyte.
The use of conventional coulometry is limited as relatively long time periods are generally required for the completion of the reactions upon which the technique relies. The reactions require mass transfer through the analyte to the working electrode of the system. Mass transfer may be assisted by mechanical stirring, but reaction times are still not satisfactory for many potential applications.
A further problem which has been encountered with conventional coulometry when applied in the water treatment industry, for example, to the determination of disinfectant residuals such as chlorine, is the possibility of competing reactions occurring at the counter electrode, such competing reactions resulting in the generation for example of oxidants. In an attempt to overcome this problem, it is known to separate the counter electrode from the analyte by an ionically conducting medium such as a salt bridge, but nevertheless the problem has deterred potential users of coulometry techniques.
The problem of slow response due to the time taken for the completion of reactions can be reduced by applying conventional coulometry techniques with an electrochemical cell in which the analyte is in the form of a thin film. Unfortunately with such an arrangement currents are generated which are a function of the structure of the electrochemical cell rather than electrochemical processes occurring in the analyte. Such currents mask the faradaic currents relevant to coulometry.